User operated drug delivery devices are as such known in the prior art. They are typically applicable in circumstances, in which persons without formal medical training, i.e., patients, need to administer an accurate and predefined dose of a medicinal product, such as heparin or insulin. In particular, such devices have application, where a medicament is administered on a regular or irregular basis over a short term or long-term period.
In order to accommodate with these demands, such devices have to fulfil a number of requirements. First of all, the device must be robust in construction, yet easy to use in terms of handling and in understanding by the user of its operation and the delivery of the required dose or medicament. The dose setting must be easy and unambiguous. Where the device is to be disposable rather than reusable, the device should be inexpensive to manufacture and easy to dispose.
Such devices, in particular pen-type injectors are typically adapted to receive a replaceable or disposable cartridge containing and providing the medicament to be disposed by the device. The cartridge comprises an outlet to be coupled with a piercing element, e.g. an injection needle, cannula or the like in a fluid transferring way. Opposite to the distal dispensing end, the cartridge is typically sealed by way of a displaceable piston slidably arranged in the body of the cartridge. In order to expel a predefined dose of the medicament, a plunger or piston rod of a drug delivery device is adapted to act on the piston for displacing said piston by a predefined distance in distal, thus dose-dispensing direction.
On the one hand, the piston has to provide a durable and leak-proof seal in order to prevent any uncontrolled discharge of the medicament. On the other hand, the piston also has to prevent ingress of impurities or other external substances into the drug receiving volume of the cartridge. Apart from its sealing capability, the piston has to be displaceable with respect to the side walls of the cartridge. Friction forces inherently present at the contact surfaces of cartridge side walls and piston should be kept to a minimum to allow for an easy and smooth operation of the cartridge.
In particular when the device and/or its cartridge are used for the first time, the dosing accuracy may be suboptimal and hence crucial. Especially prior to an initial use of the device, its mechanically interacting components may not yet fully engage and/or mutual abutment of piston rod and piston of the cartridge is just obtained only during an initial distal displacement of the piston rod. In such situations, an initial dose dispensed by the device might be too small. Moreover, since the piston sealing the vitreous and tubular shaped body of the cartridge is of a comparatively elastic and compressible material, the piston itself may experience a respective compression in axial direction during an initial displacement of the piston rod. Such an initial compression may further sustain due to frictional forces between the inside facing side wall of the cartridge's body and the lateral or circumferential side wall or respective sealing lips of the piston.
Moreover, an eventual axial relaxation of the piston is rather disadvantageous and may effectuate an increase of a fluid pressure inside the cartridge. In situations where the cartridge is still in fluid connection with e.g. a piercing element, such internal pressure may lead to a post-dispensing droplet generation to be observed at a distal tip of the piercing assembly.
Document WO 2010/133675 A1 for instance discloses a bung for drug containing cartridges having a distal and a proximal end face. This bung also comprises at least two different materials, wherein a first material covers the whole lateral area of the bung and wherein a second material is at least partly arranged inside the bung. Furthermore, the first material has a larger compressibility than the second material.
Even though this bung provides a reduced axial compressibility, manufacturing of such a piston is cumbersome and cost-intensive since the second material is entirely surrounded by the first material. Manufacturing of such a bung may for instance require a multicomponent injection moulding process.